[BitTorrent] Merkle, URLs, etc

Bill Cox bill at viasic.com
Tue Mar 8 12:44:26 EST 2005


On Sun, 2005-03-06 at 19:05 -0800, Joseph Ashwood wrote:
> ----- Original Message ----- 
> From: "John Hoffman" <theshadow at shambala.net>
> Subject: Re: [BitTorrent] Merkle, URLs, etc
...
> Any attack always affects every system, although I freely admit that the 
> hash is not the weakest link in BitTorrent security. That the current SHA-1 
> attack only finds free collisions is fairly irrelevant, attacks never get 
> worse. SHA-1 is probably good enough for now, but plans need to be put in 
> place to retire it and move to something better. It is always good to 
> prepare to simply roll over to a new primitive at a moments notice. The 
> method of doing this that I am in favor of is to prepend a primitive 
> identifier to all hashes, in the case of BitTorrent use descendency to save 
> time and simply put the hash identifier in the torrent, and implement 
> identifiers and code for SHA-245, SHA-512 and Whirlpool. This means that any 
> users who want higher security can already roll over to the stronger 
> functions right now, and at a later date when it becomes important to do so, 
> the entire system can roll over to a new hash quickly, even silently. This 
> is just good design, and will cost all of 15 bytes (maximum assuming I'm 
> counting correctly) once bencoded.
>                 Joe 

I agree.  There isn't any agreed method for extending the BT protcol,
but there are several un-official extensions floating around (like
Olaf's Merkle trees).

So, to be more exact, you'd add a new key to the .torrent files in the
'info' dictionary.  How about calling it 'hash_function'?  How about
restricting legal values for now to "SHA-1", "SHA-256" or "SHA-512"?
The nice thing about that is that we probably all already have SHA-1
through SHA-512 linked into existing clients.  If there's a reason,
Whirlpool, or any other function, could be added to the list later.

Bill




 
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